Terrestrial Planets Notes

Terrestrial Worlds in the Inner Solar System

Similarities and Differences Among Terrestrial Worlds

  • Terrestrial worlds include:
    • Mercury
    • Venus
    • Earth
    • Mars
    • Luna (Earth’s Moon)
  • These worlds possess both similar and dissimilar properties.
  • Comparative planetology: The study of planets and moons by comparing them to one another, aiming to explain these similarities and differences.

Processes Shaping Earth

  • Four primary processes have shaped Earth:
    • Tectonism: The movement of crustal plates.
    • Volcanism
    • Impacts: Craters formation due to collisions with space debris.
    • Erosion: Wearing down of surfaces by wind or water.

Tectonism

  • Tectonism: Deformation of a planet's crust.
  • Earth's crust is broken into plates.
  • Continental drift and plate tectonics describe the movement of these plates.
  • Crustal plates are moved by convection.
    • Convection: The rising and falling of hot and cold material.
  • Earth has seven major and six smaller plates.
  • Plates can separate or collide.
  • Volcanoes and earthquakes commonly occur along plate boundaries due to these motions.

Planetary Interiors

  • Deeper within a planet means higher temperature and greater pressure.
  • Formation energy and radioactive material contribute to heating the interior.
  • Smaller planets lose heat faster, while larger ones lose heat more slowly.

Volcanism

  • Volcanoes primarily form at hot spots and plate boundaries.

Planetary Magnetic Fields

  • A magnetic field acts like a giant bar magnet.
  • It originates from the motions of the iron core, though not fully understood.
  • Iron-bearing minerals reveal changes in orientation over time.
  • A sufficiently strong magnetic field is essential to prevent the solar wind from stripping a terrestrial planet of its atmosphere
  • Mars lost its magnetic field and most of its atmosphere as a result.

Impacts

  • Impact craters are created by material falling from space onto a planet's surface, experienced by all terrestrial planets.
  • Large impacts release significant amounts of energy.
  • The number of craters indicates the age of the surface.
    • An area with more craters is older than one with fewer craters.
    • This aging refers to the surface, not the entire planet/moon.
  • Tectonism and erosion can erase craters.

Meteor Terminology

  • Asteroids: Rocky/metal objects from the Asteroid Belt.
  • Meteoroid: A small cometary or asteroid fragment in space.
    • Sizes range from a speck of dust to 100 meters across.
  • Meteor: A meteoroid that enters and burns up in an atmosphere.
  • Meteorite: Any meteoroid that survives to hit the ground.

Types of Meteorites

  • Stony meteorites: Mostly silicate minerals, hard to distinguish from Earth rocks.
    • Comprise 75-90% of all meteorites.
  • Iron meteorites: Mostly iron and nickel, easiest to spot and find.
    • Comprise 10-25% of all meteorites.
  • Stony-iron meteorites: A combination of stony and iron materials.

Erosion

  • Erosion includes processes that wear down high spots and fill in low spots.
  • Wind and water strongly erode features.

Water

  • Earth is the only terrestrial planet with liquid water today.
  • Water modified the surface of Mars in the past and exists today as ice.
  • Water ice exists on the Moon and maybe Mercury.

Mercury (☿)

  • Closest planet to the Sun.
  • Named after the Roman Messenger God.
  • Smallest of the terrestrial planets.
  • Lacks an atmosphere.
  • Experiences extreme temperature variations:
    • 800°F (427°C) facing the Sun.
    • -279°F (-173°C) facing away from the Sun.

Geology of Mercury

  • Geologically active with an active magnetic field.
  • Surface shrank after cooling, deforming it and leaving kilometer-high cliffs called scarps.
  • Surface is covered in impact craters.
  • Also has smooth surfaces from past volcanism, with a few inactive volcanoes identified.

Venus (♀)

  • Second planet from the Sun.
  • Named after the Roman God of Beauty.
  • Nicknamed “Earth’s twin” due to similar size.
  • Has extremely high surface temperature: 870°F (465°C), hot enough to melt lead.

Geology of Venus

  • Venus has the largest number of volcanoes in the Solar System, some active.
  • Lacks tectonism; volcanoes formed like those in Hawaii (hot spots).
  • Wind modifies the surface and extremely heavy atmosphere with 92 bars of pressure (Earth has 1 bar).

Venus’ Strange Magnetic Field

  • Venus has a weak magnetic field induced by the solar wind interacting with the planet’s ionosphere.
  • This field isn’t produced internally like Mercury and Earth.
  • The weak field results in substantial atmospheric stripping.

Earth (♁ or ⊕)

  • Third planet from the Sun.
  • Exists inside the goldilocks zone of the solar system.
  • Only planet known to have life.

Modeling Earth's Interior

  • We model Earth’s interior by studying earthquakes.
  • Seismic waves travel differently through different materials.
    • Primary waves: Travel through solids and liquids.
    • Secondary waves: Travel through solids only.

Earth’s Interior Structure

  • Consists of:
    • Crust (only Earth has its crust broken up into plates)
    • Mantle
    • Core (dense materials)
  • Differentiation: Dense materials sink, low-density materials rise.

Earth’s Magnetic Field

  • Earth has a substantial magnetic field.
  • Protects the Earth from the Solar Wind.
  • Produces the Auroras.
    • Charged particles from the solar wind interacting with gas in Earth’s upper atmosphere.

Impacts on Earth

  • Barringer Crater in Arizona:
    • Formed by the impact of a large meteorite (300,000 tons, 150 feet across).
    • Impact energy equivalent to 250,000,000 tons of TNT (150 times the nuclear bomb dropped on Hiroshima).
  • Mass Extinction Events:
    • About 65 million years ago, a 10 km-wide meteoroid struck the Yucatan Peninsula.
    • Caused massive climate changes, leading to the extinction of the dinosaurs.
    • Iridium found in a layer of soil all over the world is the “smoking gun”.

Impact Frequency

  • Venus and Earth have relatively few craters compared to Mercury and the Moon.
  • The energy released by a 100 kg meteoroid is equivalent to 10 tons of dynamite.

Luna (☾) - Earth’s Moon

  • Earth’s Moon.
  • Was once geologically active and even had a magnetic field.
  • Source of the tides on Earth (along with the Sun).

Formation of the Moon

  • Formed in a large collision between Earth and a Mars-sized protoplanet.
  • The material collected to form the Moon.
  • The composition of the Moon is similar to that of Earth’s crust.

Geology of the Moon

  • Geologically Inactive.
  • The Moon lost its magnetic field and volcanism due to heat loss.
  • It does not have any volcanoes, but lava flows smoothed out parts of its surface.
  • The side facing Earth is covered in lunar maria, resulting from ancient lava flow.
  • Covered in impact craters (most from the Late Heavy Bombardment).

Mars (♂)

  • Fourth planet from the Sun.
  • Named after the Roman God of War.
  • Exists inside the goldilocks zone of the solar system.
  • Only terrestrial planet other than Earth to have moons (Deimos & Phobos).

Geology of Mars

  • Geologically dead with no active magnetic field.
  • Mars has experienced extensive tectonism and boasts the massive chasm Valles Marineris.
    • 4000 km long, 200 km wide, 7 km deep.
  • Wind modifies the surface.
  • Used to experience water erosion.

Mars’ Former Volcanism

  • The volcanoes on Mars are the largest mountains in the Solar System.
  • Formed similarly to volcanoes in Hawaii.
  • Example: Olympus Mons (largest volcano in the solar system!).